Catalytic converter assemblies for treating exhaust gases of automotive and diesel engines contain a fragile structure, such as a catalyst support structure, for holding the catalyst, used to effect the oxidation of carbon monoxide and hydrocarbons and the reduction of oxides of nitrogen, the fragile structure being mounted within a metal housing. The fragile structure is preferably made of a frangible material, such as a monolithic structure formed of metal or a brittle, fireproof ceramic material such as aluminum oxide, silicon dioxide, magnesium oxide, zirconia, cordierite, silicon carbide and the like. These materials provide a skeleton type of structure with a plurality of tiny flow channels. However, as noted hereinabove, these structures can be, and oftentimes are, very fragile. In fact, these monolithic structures can be so fragile that small shockloads or stresses are often sufficient to crack or crush them.
The fragile structure is contained within a metal housing, with a space or gap between the external surface of the fragile structure and the internal surface of the housing. In order to protect the fragile structure from thermal and mechanical shock and other stresses noted above, as well as to provide thermal insulation, it is known to position at least one ply or layer of mounting or support material within the gap between the fragile structure and the housing. For example, assignee's U.S. Pat. Nos. 4,863,700, 4,999,168, 5,032,441, and 5,580,532, the disclosure of each of which is incorporated herein by reference, disclose catalytic converter devices having a mounting or support material disposed within the gap between the housing and the fragile structure contained in the devices to protect the fragile structure and otherwise hold it in place within the housing.
However, even some of the latest mounting materials used in these catalytic converter devices, while suitable for most current automotive catalytic converters, do not entirely satisfy all design requirements of the fragile structure and catalytic converter manufacturers. In particular, the residual holding pressure exerted by many of the state-of-the-art plies of support material, often referred to as mounting mats, have been found to be inadequate at times where the catalytic converter has undergone wide temperature fluctuations, thereby causing significant expansion and contraction of the metal housing in relation to the fragile structure, also referred to as the catalyst support structure, which, in turn, causes significant compression and expansion cycles for the mounting mats over a period of time. It has been found that these state-of-the-art mounting mats do not sufficiently hold the fragile structure in the most severe of applications where temperatures reach well above 900° C., and often undergo constant thermal cycling. Vibration and mechanical shock are further problems for the mounting mats.
These problems are even further amplified in catalytic converter systems having catalytic support structures greater than 35 mm in diameter because a larger structure implies a larger outer housing which, in turn, implies a larger gap expansion between the fragile support structure and housing at elevated temperatures due to the larger thermal expansion of the housing with respect to the fragile support structure. Under normal operating conditions, a minimum effective holding pressure for the support element or mat of approximately 2 psi is sufficient to prevent the fragile structure from being dislodged and damaged. The effective mat holding pressure is defined as the mat holding pressure multiplied by the coefficient of friction of the mat/fragile structure interface. The coefficient of friction of typical mat products is approximately 0.45 in the in-use condition. The mounting mat, therefore, is required to have a sufficient residual minimum holding pressure after 200 cycles of testing at a nominal temperature of 900° C. of at least 4 psi, and after 1000 cycles of testing at a nominal temperature of 900° C. of at least 4 psi. More preferably, the support element (i.e., mounting mat) should have a minimum holding pressure after 200 cycles of testing at the nominal 900° C. temperature of at least 10 psi. When tested after 1000 cycles at 900° C., the support mat should more preferably have a minimum holding pressure of at least 6 psi, and even more preferably, at least 12 psi. Still further, the support element should exhibit predictable and acceptable degradation with high temperature exposure and mechanical cycling, meaning the ply or plies should preferably exhibit a regular pattern of degradation of no more than about 1 psi per 100 cycles after about 600 cycles.
Most of the mounting mats, heretofore, have attempted to overcome the degradation and thermal cycling problems by using extremely expensive, high alumina refractory ceramic fibers which add significantly to the cost of the production of the mounting mat. These refractory ceramic fibers are made from an aqueous solution or a colloidal dispersion that is called an “organosol” or a “sol gel”. While ceramic fibers formed by sol gel processes may offer a high degree of resiliency needed for mounting monolithic structures, their high cost have forced manufacturers to seek other, less expensive solutions.
For instance, some manufacturers of mounting or support mats have resorted to expensive preprocessing such as stitch binding prior to installation of the mat. In other instances, the mounting material used may be required to be used in combination with other mounting materials, such as intumescent sheets and backing layers, in order to provide sufficient strength for handleability and for resiliency. These mounting materials are generally very thick and lack the structural integrity necessary, and may even require being handled in a bag to prevent crumbling. The mounting materials are also difficult to cut to size for installation, and further must be compressed substantially to fit enough material needed for supportive mounting within the gap between the catalyst support structure and the housing. Consequently, “flashing” commonly occurs, with excess material being squeezed out of the housing.
As an alternative to using sol gel-derived, refractory ceramic fibers, attempts have been made to form the ceramic fibers using melt processing techniques. However, conventional melt-formed ceramic fibers typically contain shot, on the order of 30 to 60 percent, and have been deemed not suitable for the particular application of a mounting mat within a catalytic converter or other similar device. Although it is possible to treat the fibers to reduce the shot content to as low as 5 percent, at least some patents, such as U.S. Pat. Nos. 4,929,429, 5,028,397 and 5,250,269, have suggested that these treated fibers still lack the requisite resiliency and, therefore, are not capable of providing the necessary holding pressure at a nominal temperature of 900° C.
However, at least one patent has attempted to overcome these shortfalls with melt-formed refractory ceramic fibers. U.S. Pat. No. 5,250,269 teaches that a mounting mat may have the requisite resiliency values if it is produced using a particular annealing process to form substantially amorphous refractory ceramic fibers for the mat. By “substantially amorphous” is meant that no crystallinity can be detected by x-ray diffraction. In order to obtain this result, U.S. Pat. No. 5,250,269 specifies that annealing temperatures of at least 700° C. and less than 990° C. are required to achieve a substantially amorphous melt-formed ceramic fiber. It is suggested that the annealing process provides for suitable ceramic fibers having sufficient resiliency values, regardless of the amount of shot contained therein. The particular type of melt processed fibers employed, i.e., melt blown fibers or melt spun fibers, is not disclosed in U.S. Pat. No. 5,250,269.
In British Patent Specification No. 1,481,133, it is suggested that a blanket or sheet of amorphous ceramic fibers will retain a substantially permanent set under compression, but that good resiliency can be achieved by converting the fibers from an amorphous form to a fine-grained crystalline form having a crystalline size of less than 200 Å, after which the blanket will return to about 85 to 90% of its original configuration after compression. This can be achieved, according to the British specification, by heating the fibers to a temperature above the devitrification temperature of about 950° C., while avoiding higher temperatures above about 1050° C., since higher temperatures are recognized as forming course-grained structures which, according to the British specification, produces poor handling properties. The fibers are heated at the above-noted temperatures for a period of time sufficient to produce devitrification throughout the refractory ceramic fibers, but must be terminated prior to the onset of excessive grain growth. According to the British specification, such a time period may vary from 10 minutes to up to 1 hour.
Although the British specification characterizes the fibers as capable of returning to at least 85 to 90 percent of its original configuration when a compression force is released, the specification does not specify what applications are suitable for such a blanket or sheet of fibers, although mats and blankets of refractory ceramic fibers were commonly used in the 1970's as furnace liners. There is no mention whatsoever of the fibers being suitable for use in the mounting mats of catalytic converters.
The present invention seeks to use high index, crystallized, melt-formed ceramic fibers by heat treating them at temperatures above the mullite crystallization temperature of 980° C., and more preferably, at temperatures ranging from 990° C. to about 1400° C. in a controlled manner to obtain specific amounts of crystallinity and crystallite size, thereby increasing fiber performance in the form of a catalytic converter support mat. Such fibers will desirably have at least about 5 to about 50 percent crystallinity as detected by x-ray diffraction, and a crystallite size of from about 50 Å to about 500 Å. When such fibers are employed, the support mat provides a minimum pressure for holding the fragile catalyst support structure within the housing of at least one of i) at least 4 psi after at least 200 cycles and/or after 1000° C. of testing at 900° C. or ii) at least about 10 psi after at least 1000 cycles of testing at 750° C.